The present invention relates to a resonant sensor, in particular to a resonant pressure sensor, in particular to a resonant pressure sensor of the type which is highly accurate and is used for detecting small changes or variations in barometric pressure.
Resonant pressure transducers of the type which comprise a xe2x80x9cbutterflyxe2x80x9d-shaped resonator defined by a boron etch stop have been in manufacture for several years and have demonstrated excellent performance. These sensors are known as xe2x80x9cRPTxe2x80x9d resonant pressure transducers.
In a paper entitled xe2x80x9cA high accuracy resonant pressure sensor by fusion bonding and trench etchingxe2x80x9d by Welham et al, published at Sensors and Actuators 76 (1999) p298-304, a novel design of pressure sensor based upon an electrostatically driven and piezoresistively sensed xe2x80x9cdouble shuttlexe2x80x9d lateral resonator is presented.
The sensor described in that paper is manufactured from a square of silicon, using a trench etching technique. Silicon is a particularly advantageous material to use, since it has a high gauge factor, i.e. it has a high change of resistance with strain. The sensor described in the paper comprises a single crystal silicon resonator, suspended between two pedestals which are an integral part of a diaphragm. The design of the diaphragm is based on the known RPT design, where the pedestals move apart approximately 2 xcexcm at a full scale pressure of 3.5 bar. The xe2x80x9cdouble-shuttlexe2x80x9d resonator is formed from two inertial masses and eight flexures. The resonator is excited electrostatically into a balanced mode of oscillation via a pair of comb capacitors. Its motion is sensed via a pair of piezoresistors that couple together each inertial mass. Upon the application of a pressure, the diaphragm deforms, so stressing the resonator and inducing a shift in its resonant frequency.
The structural layer which forms the resonator, conductive tracks and pads is electrically isolated from the diaphragm by an oxide layer. The tracks are curved when bridging across to the resonator to minimise their contribution to the stiffness of the diaphragm. An encapsulation chip is bonded over the resonator to allow operation in vacuum and to protect the resonator from the enviromnent, e.g., dust, corrosive chemicals and condensation.
While the design of sensor described in the paper has a number of advantages over the RPT sensor, it also has a number of disadvantages. In particular, although the strain gauge has a high output compared to the RPT sensor, it is desirable, for particularly demanding applications, to increase this still further. Additionally, heat is generated within the strain gauge and the physical location of the strain gauge close to the centre of the sensor, means that measures have to be taken to ensure adequate heat dissipation. Finally, the design of the sensor requires that electrical connections are provided between stationary and moving parts of the sensor.
It is an object of the present invention to provide a sensor in which the above disadvantages are reduced or substantially obviated.
The present invention provides a resonant sensor which comprises a support structure comprising two support points; a laminar resonator suspended between said two support points of said support structure and comprising a plurality of substantially parallel flexural members which are responsive to relative movement of said support points; means for exciting said resonator into a balanced mode of oscillation and means for sensing motion of said resonator wherein said means for sensing motion of said resonator is or are spaced from, and linked to, said flexible area of said resonator by means of levers.
Said support structure preferably comprises a diaphragm and two pedestals formed integrally with said diaphragm.
Said laminar resonant sensor preferably comprises a structural layer electrically isolated from said diaphragm and comprising a single crystal silicon resonator suspended between said pedestals.
Said means for exciting said resonator preferably comprises means for exciting said resonator electrostatically, more preferably a pair of comb capacitors. In a further preferred embodiment of a resonant sensor according to the present invention said means for sensing motion of said resonator comprises at least one piezoresistor, which piezoresistor is preferably mounted on or adjacent to the outer edge of said sensor.
Said single crystal silicon resonator is preferably substantially rectangular in shape and said flexural members are preferably formed in said central portion of the rectangle, more preferably by the technique of trench etching. Said structural layer is preferably electrically isolated from said diaphragm by an oxide layer.
Said support points are preferably adapted to move relative to each other in response to a difference in pressure, force or acceleration.
In a particularly preferred embodiment of a resonant sensor according to the invention, said support points are adapted to move relative to each other in response to a difference in pressure.